For a portable electronic device equipped with a hard disk drive (HDD), it is important that the electronic device is protected from a shock in the event it falls onto the ground. The HDD writes and reads data to and from a magnetic disk with a head slider flying over the rotating magnetic disk. Therefore, when a shock occurs due to the falling of the HDD in operation, the head slider crashes against the magnetic disk so that the magnetic disk or a head element portion on the head slider might be damaged.
As a protection mechanism to prevent such a damage of the HDD, a mechanism that detects the HDD or the electronic device equipped with the HDD being in a falling state and retracts the head slider to a retract position has been known in the art. For example, Japanese Patent Publication No. 2005-235382 (“Patent Document 1”) discloses a technique which detects the shock amount applied to the HDD and retracts the head slider if the shock amount exceeds the critical value. Patent Document 1 also discloses a technique that changes the retract position of the head slider to the inner peripheral side or the outer peripheral side according to the position of the head slider.
Specifically, in Patent Document 1, the HDD detects the applied shock amount and determines whether the detected shock amount exceeds the critical value or not. Then, if the detected shock amount exceeds the critical value, it reads the positional information of the cylinder at which the head element portion on the head slider is located, and stops the command in execution. The HDD determines whether the read cylinder number exceeds a reference cylinder number or not, and moves the head slider toward the innermost peripheral direction of the disk if the read cylinder number exceeds the reference cylinder number, or otherwise moves the head slider toward the outermost peripheral direction of the disk.
In order to protect the magnetic disk or the head element portion from a shock by a fall, it is preferable to retract the head slider quickly to the retract position before the shock arises. Therefore, it is important to shorten the time to retract the head slider to the retract position upon the determination of the fall.
The technique of the above Patent Document 1 attempts to shorten the retract time by changing the retract position according to the current position of the head slider. However, the technique of the above Patent Document 1 starts retracting after the HDD receives the shock so that the retracting may not be in time. Besides, there is a limit to shorten the retract time because the retract time depends on the initial position of the head slider. Moreover, the retract positions are needed to be formed at the both of the outer and inner peripheral ends of the magnetic disk, which results in reducing the storage capacity of the disk.
In order to give a priority in protecting the magnetic disk and the head element portion from the shock by the fall, it can be considered that the determination reference for the fall is relaxed. However, if the fall determination reference is relaxed too much, a problem is caused that erroneous determinations of falls frequently occur although actual falls do not occur. Because the head slider cannot access the magnetic disk while the head slider is positioned at the retract position and much time is required for the head slider to move from the retract position to the initial position, the performance of the HDD goes down very much if the erroneous determinations increase. This is particularly apparent in HDDs with a load/unload scheme. Therefore, it is required to reduce the erroneous determinations of falls in the HDDs.